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 AN INTRODUCTION TO WELL TESTING

Well testing is the science of simultaneously measuring a well’s flowrate and corresponding pressure response, to determine well and reservoir parameters. In short, a well test gives you a look at the reservoir, which can provide valuable insight into what a well, or even an entire field, is capable of producing. While there are many different types of well testing techniques, there are three common methods which will be discussed below: Drawdown/Buildup Tests, Multi-Rate Deliverability Tests, and Interference Tests.

Drawdown/Buildup Test

The most common form of well testing is a drawdown/buildup test, which is also referred to as a “pressure transient test”.  This is performed by opening a shut-in well and flowing it at a constant flowrate until the pressure stabilizes (drawdown), thus indicating that the well is in pseudo-steady state flow.  A pressure buildup then follows the drawdown portion of the test. This is done by flowing a well for a set period of time and then shutting the well in for an equivalent period of time, while monitoring the pressure (buildup). This allows for a detailed analysis, which can include permeability, average reservoir pressure, reservoir temperature, and skin factor. The skin factor quantifies the amount of stimulation or damage that is present in the reservoir near the wellbore. In some cases, if the test is run long enough, it can also provide valuable information about the reservoir description including the presence of natural fractures, layering, distance to boundaries and even reservoir shape.

It is important to note that the same information can be obtained from an injection-falloff test. In that type of test the injection period is analogous to a drawdown period, and the pressure falloff period is analogous to a pressure buildup period.

Multi-Rate Deliverability Test

A multi-rate deliverability test, often referred to as an “isochronal test”, requires that a well be flowed and shut in at several different flowrates (typically 3 to 4 rates), with buildup periods between each rate. It is important that each flowrate, and corresponding buildup last the same length of time. As an example, the flow and buildup periods might look as follows:

 1.      Flowrate #1 – 1 hour

2.       Buildup #1 – 1 hour

3.       Flowrate #2 – 1 hour

4.       Buildup #2 – 1 hour

5.       Flowrate #3 – 1 hour

6.       Buildup #3 – 1 hour

7.       Extended Flowrate #4 – 2 hours

8.       Extended Buildup #4 – 2 hours

 The final rate and buildup periods are typically extended to ensure that pressure stability exists prior to terminating the test. If pressure stability has been achieved in the previous rates, there is no need to run the final rate longer.

A multi-rate test has two key advantages over a single drawdown/buildup test. First, there are three different flow periods which can be analyzed, and thus the permeability, skin and reservoir boundaries can be calculated multiple different times to help validate the results and provide confidence in the analysis. Secondly, by having three separate and distinct flowrates, C and n values can be calculated, which yields an accurate deliverability equation for the well. The deliverability equation is as follows:

Where:                 Q = Flowrate

                                C = Flow Coefficient

                                Pres = Shut-In Pressure (Reservoir Pressure)

                                Pwf = Well Flowing Pressure (Stabilized Pressure)

                                n = Deliverability Exponent

Interference Test

A common concern, particularly within high-permeability reservoirs, is well communication between offset wells.  An interference test is a method for determining the level of communication between wells and quantifying their interactive effects on production from a nearby well. This type of test is performed by flowing one well, while measuring the pressure response in a nearby well. If the wells are in communication, a pressure response from the flowing well will be seen in the shut-in well nearby. This is valuable when planning well spacing for field development. It is also valuable in storage fields for determining field deliverability when a well is taken offline.

Another benefit of an interference test is that it can provide more accurate reservoir parameters in instances where the parameters are heterogenous. For example, porosity is typically determined from open-hole logs, or from core data (if it exists) but this method only provides the porosity at a single point in the reservoir. An interference test can give insight into the average porosity between two wells, which may be different than what was determined from logs or cores. Because porosity is a major factor in reservoir volume, a more accurate determination of average porosity will lead to more accurate volumetric calculations.

Summary

There are many different types of well testing methods, and each of the three methods mentioned can have many different variations. Every well, reservoir and situation are unique, and thus every test should be designed with the specific goals in mind. High-quality pressure and flowrate data are absolutely necessary to ensure that a reliable well test interpretation can be performed.

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